An optical time-domain reflectometer (OTDR, Optical Time-Domain Reflectometer) is made according to the backscattering and reflection principles of light, where attenuation information is obtained by using backscattered light produced during light propagation in an optical fiber, and may be used to measure optical fiber attenuation and splice loss, locate an optical fiber fault point, and learn the loss distribution along the optical fiber. Because of the uneven density of optical fiber materials, uneven doping components, and defects of the optical fiber itself, when light is transmitted in the optical fiber, every point along the optical fiber will cause scattering. The optical time-domain reflectometer records the strength of scattered light which is collected at every time point. The speed of light is constant, and there is a mapping relationship between the time when a signal is collected and a transmission distance of light in the optical fiber; therefore, the time may be converted into the length of the optical fiber. FIG. 1 shows a typical expression form of an OTDR detection signal curve, where the horizontal axis indicates the length of an optical fiber (in km), and the vertical axis indicates the relative strength of scattered optical signals (in dB). It can be seen that, the height of the curve 100 gradually decreases as the length of the optical fiber increases; however, the change tends to be smooth. 104 indicates the strength difference of OTDR detection signals at two points resulting from different lengths of the optical fiber. The curve 100 changes significantly at the splice point (Splice) 101, optical fiber connection box (Connection) 102, and break point (break) or end of optical fiber (End of fiber) 103, which indicates that light is reflected or scattered at these places. The scattered light and reflected light may be partially transmitted back to the optical time-domain reflectometer. The areas of significant changes (105, 106, and 107) of the curve in FIG. 1 are referred to as reflection events and attenuation events. Transmission characteristics of the optical fiber at the places may be determined depending on the strength changes of the received light.
The conventional OTDR measurement principle is as follows: A pulse optical signal with a controllable width is coupled into an optical fiber; during the transmission process of the pulse light in the optical fiber, as scattering occurs, part of scattered signals in a direction opposite to the pulse transmission finally return to the OTDR. The OTDR receives the signals by using a coupler and performs analog-digital conversion to convert the signals into digital signals. The digital signals are converted into a curve where the length of the optical fiber (in km) is the horizontal axis and the relative strength (in dB) is the vertical axis. The curve is a straight line which starts from the origin of the optical fiber and gradually attenuates as the length of the optical fiber increases. However, due to phenomena of splices and breaks in the optical fiber, additional loss and reflection occur at these places, and the expression form thereof is shown by the reflection events and attenuation events in FIG. 1. OTDR measurement analyzes the state of an optical fiber link depending on the curve 100 having reflection and attenuation events. An exception is indicated if there is too strong reflection or too much loss at some places; because the horizontal axis of the curve is the length of the optical fiber, the places of the reflection and attenuation events may be calculated.
The OTDR measures the scattered signal of light. The scattered signal of the light is related to the peak power and pulse width of the detection pulse light, and decreases as the transmission distance increases. The scattered signal of the light is very weak in strength and may easily be covered by noise, which limits the detection distance of the OTDR. The detection distance corresponds to an indicator “dynamic range.” “Dynamic range” is used to represent the maximum detection distance of the OTDR. The “dynamic range” is commonly defined by a dB difference between a backscattered signal level at the origin and a peak level of noise. Common methods for improving the dynamic range are increasing averaging times, increasing the detection pulse width, and using digital filtering technologies.
In order to obtain a better signal-noise ratio (that is, to improve the dynamic range and extend the detectable distance), normally, the multi-time accumulation technology (also referred to as averaging) is used. The implementation process of the multi-time accumulation technology is as follows: An OTDR controls a laser to send a pulse signal into an optical fiber. The pulse signal constantly produces backscattered signals during transmission in the optical fiber, which return to the OTDR with a coupler. The OTDR constantly receives the backscattered signals since the moment the pulse is sent. The OTDR performs photoelectric conversion, signal amplification, and analog-digital conversion on the received backscattered signals and stores them. Normally, this process continues until a scattered signal produced by the detection pulse signal at the end of the optical fiber returns to the OTDR instrument, that is, a period twice the time when the detection pulse is transmitted in the entire optical fiber (because there is a process for the backscattered signal to return, the transmission time doubles that for forward transmission). This process is referred to as OTDR sampling. This process is repeated and data obtained at multiple sampling times is accumulated and averaged, which may suppress white noise and improve the signal-noise ratio of OTDR signals. FIG. 2a shows an OTDR measurement result with a small number of averaging times, and FIG. 2b shows an OTDR measurement result with a large number of averaging times. The horizontal axes of the two figures represent the length of an optical fiber (in km), and the vertical axes are the relative strength (in dB) of sampled data. By comparing FIG. 2a with FIG. 2b, it can be seen that the locations where areas of sharp changes occur in FIG. 2a and FIG. 2b are basically the same; however, in FIG. 2b, the differences between the areas of sharp changes and other parts are more obvious, indicating that the dynamic range is improved after the number of averaging times is increased.
FIG. 3 is a schematic structural block diagram of an OTDR. A pulse generator 303 transmits a narrow pulse of an adjustable width to drive a laser 301, and the laser 301 generates pulse light of a required width. In FIG. 3, the graph above the arrow from the laser 301 to a coupler 302 is a schematic waveform of the pulse light. The pulse light goes through the coupler 302 for directional coupling and then enters an optical fiber 308 under test. Backscattered light and Fresnel reflected light in the optical fiber 308 go through the coupler 302 and enter a photoelectric detector 305. The photoelectric detector 305 converts the received scattered optical signal and reflected optical signal into electrical signals, which are amplified by an amplifier 306 and then sent to a signal processing component 307 for processing (including a sampling unit, an analog-digital conversion unit, and an averaging unit). The processing result is displayed by a displaying unit 309, where a vertical axis represents a power level and a horizontal axis represents a distance. A time base and controlling unit 304 controls the width of the narrow pulse of an adjustable width transmitted by the pulse generator 303, and controls the sampling of the sampling unit and the averaging of the averaging unit in the signal processing component 307.
A submarine cable monitoring device is a device for performing routine maintenance and fault location for submarine cables, and also uses the OTDR technology. A submarine cable monitoring device sends detection light into a submarine cable, and detects operating states of a submarine cable and a submarine device such as a repeater by using the received Rayleigh backscattered signal of the detection light. In a different scenario, a submarine cable system has a specific value range limit on the detection light power and signal pulse width, and the signal pulse width must be limited within a specific range in order to obtain effective monitoring accuracy. Therefore, how to obtain a bigger dynamic range and higher monitoring accuracy under the circumstance of limited detection signal power and pulse width becomes a difficult problem to be solved for the submarine cable monitoring device.
Because a submarine cable system is a cascade system having multiple repeaters, it has optical fiber transmission in two directions, namely, uplink and downlink, and a repeater amplification system. Every repeater of the submarine cable system has a loopback function, which ensures that a backscattered signal generated by a detection pulse can be coupled into a reverse transmission line and be sent back to the submarine cable monitoring device. The expression form of an OTDR signal in the submarine cable system is shown in FIG. 4, where the horizontal axis is the length of the submarine cable and the vertical axis is the relative strength of the OTDR signal. The place of every peak value corresponds to a repeater and an amplifier. The maximum detection distance of the OTDR signal is 600 km. Hence, the signal at the distance of 600 km is expressed as noise.
The OTDR described above performs detection by using single-pulse detection light. If the OTDR extends the single-pulse detection light into pulse sequence detection light and further uses the correlation between pulse sequences, it is referred to as a correlation OTDR (Co-relation OTDR). The correlation OTDR transmits the pulse sequence detection light and performs a correlation operation on a received scattered signal. Such correlation processing may effectively improve the signal-noise ratio of the received signal. By transmitting a pulse sequence, the signal-noise ratio may be improved under the circumstance that a single pulse in the detection pulse light sequence is narrow enough, thereby effectively solving the conflict between an optical fiber event resolution and the dynamic range, and improving the detection performance.
FIG. 5 is a schematic structural block diagram of a correlation OTDR in the prior art, where solid line arrows between modules represent optical signals and dashed line arrows between modules represent electrical signals. A code pattern generator 505 is configured to generate a pulse sequence (the graph above the arrow from the code pattern generator 505 to a correlation processing unit 507 is a schematic diagram of the waveform thereof). The pulse sequence is sent to a modulator 502 which modulates laser light transmitted by a laser 501 into pulse sequence detection light (the graph above the arrow from the modulator 502 to a coupler 503 is a schematic diagram of the waveform thereof). The pulse sequence detection light goes through the coupler 503 for directional coupling and then enters an optical fiber 504. The reflected light and scattered optical signal collected by the coupler 503 are converted into an electrical signal by a photoelectric detection unit 506, processed by the correlation processing unit 507, and then output to a display device (not shown in FIG. 5) for displaying the analysis result (the graph above the arrow to the right of the correlation processing 507 is a schematic diagram of the waveform thereof).
The biggest difference of the measurement principle between the correlation OTDR and the conventional OTDR described above lies in that, in this technology, instead of a pulse, a pulse sequence is transmitted in “every sampling process.” The pulse sequence is designed for correlation operation. At present, the most popular correlation code pattern is Gray codes which consist of four groups of codes. The Gray codes are expressed as code strings consisting of 0 and 1 in number. The signal transmitted by a laser is continuous light. Under the control of a code pattern generator, a modulator modulates the continuous light into a form of a group of Gray code sequences. This group of Gray codes goes through a coupler and then enters an optical fiber for transmission. A receiving and sampling process continues until this group of codes is completely transmitted from the end of the optical fiber and the backscattered signal thereof completely returns to an OTDR instrument, thereby completing “one sampling process” of “one group of Gray codes”. Normally, four groups of Gray codes need to undergo such a process of sampling sequentially, and finally four groups of sampled data are obtained. Correlation operation is respectively performed on the four groups of sampled data with the digital Gray codes generated by the code pattern generator, and data is regrouped, which still can be restored into the backscattered signal form obtained by a conventional OTDR sending a single pulse. The difference lies in that the digital correlation processing is capable of suppressing noise and improving the signal-noise ratio, and the OTDR signal is improved.
The disadvantage of the existing correlation OTDR technology lies in a heavy operation load, making it difficult to implement real-time processing. As described above, four groups of Gray codes need to be respectively transmitted, received, and stored, and a correlation operation is performed after the detection signals of the four groups of codes are completely received for restoring to OTDR signals. In a submarine cable system, normally the total length of cables is very large (some cables may reach a length of 12000 km), and a huge amount of data needs to be processed. In particular, some correlation code patterns require hundreds of code groups to be transmitted. Under such a circumstance, it is difficult for a submarine cable monitoring device to realize real-time processing and display states of submarine cables in real time.